Simple JavaScript stubbing function

_StubbingFunction.js

function stub() {
    return {
        of : function (name, callback, returnValue) {
            this[name] = function () {
              var args = Array.prototype.slice.call(arguments);
              this[name].calls.push(args);
              var ret = null;
              if(callback)
                  ret = callback.apply(this, args);
              if(returnValue) return returnValue;
              return ret;
          };
          this[name].calls = [];
 
          return this;
        }
    };
}

HTMLCanvasExample.js

function renderBlueSquareAndTranslate(height,width, context) {
    context.fillStyle = "#0000FF";
    context.translate(50,0);
    context.fillRect(0,0,height,width);
}

test("will render blue square and translate right 50px", function() {
   var context = stub().of("fillRect")
                       .of("translate");
 
   renderBlueSquareAndTranslate(10,10, context);
 
   equal(context.fillRect.calls.length, 1);
   equal(context.translate.calls[0][0], 50);
   equal(context.translate.calls[0][1], 0);
   equal(context.fillStyle, "#0000FF");
});

StubbingFunctionTests.js

test("will capture call arguments for multiple calls", function() {
   var s = stub().of("action");
 
   s.action("matt",8);
   s.action("mal",4);
 
   equal(s.action.calls.length,2);
   equal(s.action.calls[0][0],"matt");
   equal(s.action.calls[0][1],8);
   equal(s.action.calls[1][0],"mal");
   equal(s.action.calls[1][1],4);
});

test("will invoke callback with arguments", function() {
   var arg0, arg1;
   var callback = function() { arg0 = arguments[0]; arg1 = arguments[1]; return 10;};
   var s = stub().of("action", callback);
 
   var res = s.action("matt", 8);
 
   equal(arg0,"matt");
   equal(arg1,8);
   equal(res,10);
});

test("will use returnVal over return value of callback", function() {
   var arg0, arg1;
   var callback = function() { arg0 = arguments[0]; arg1 = arguments[1]; return 5;};
   var s = stub().of("action", callback, 10);
 
   var res = s.action("matt", 8);
 
   equal(res,10);
});

test("will stub multiple methods and test only one is called", function() {
   var s = stub().of("action").of("other");
 
   s.action();
 
   equal(s.action.calls.length,1);
   equal(s.other.calls.length,0);
});